1. Field of the Invention
The present invention relates to a method of manufacturing a laminated ring for use in a continuously variable transmission, and an apparatus for measuring the circumferential length difference of a ring in such a method.
2. Description of the Related Art
Continuously variable transmissions have a power transmitting belt trained around a pair of pulleys. The power transmitting belt comprises a laminated ring composed of a plurality of rings and mounted on and held by an element of predetermined shape.
The laminated ring is straight in shape when traveling between the pulleys, and curved in shape when running along the pulleys. The laminated ring undergoes severe bending deformations due to repetitive cycles of the straight and curved states. Therefore, the laminated ring is required to have a mechanical strength large enough to withstand the severe bending deformations.
One known material capable withstanding such severe bending deformations is maraging steel. The maraging steel is a low-carbon steel containing 17 to 19% of Ni, and Co, Mo, Ti, etc. When the maraging steel is heated to a suitable temperature after being subjected to a solution treatment, it causes age hardening in a martensitic state, resulting in an ultra-high strength steel that is highly strong and highly tough. The maraging steel is highly suitable for use as the material of the laminated ring.
The laminated ring has heretofore been manufactured according to the following process: The ends of a thin sheet of maraging steel that is an ultra-high strength steel are welded to each other, producing a cylindrical drum. The cylindrical drum is subjected to a first solution treatment in order to uniformize the hardness that has been partly increased due to the heat applied when the thin sheet of maraging steel was welded. Then, the cylindrical drum is severed into rings of certain width, which are rolled to a predetermined target circumferential length. The rings are then subjected to a second solution treatment in order to recrystallize the rolled structure for restoring the metal crystal grain configuration that has been deformed by the rolling process.
The rings are rolled such that the circumferential lengths of the rolled rings will be equalized to the predetermined target circumferential length. Actually, however, the circumferential lengths of the rolled rings suffer an error as a difference from the target circumferential length. Therefore, the rings that has been subjected to the second solution treatment are corrected for the target circumferential length, and then aged and nitrided for increased hardness. The rings of slightly different circumferential lengths are fitted and laminated together into a laminated ring.
The rings of certain width which have been severed from the cylindrical drum are in the shape of an endless ribbon. When a ring thus shaped is trained around a pair of rollers and rolled by increasing the diameter of the ring under certain tension, a circumferential length difference is developed between opposite circumferential edges of the ring, causing the ring to take on a tapered cross-sectional shape. More specifically, since each of the rollers is supported at one end of its shaft or in a cantilevered fashion, the other ends of the shafts of the respective rollers tend to be tilted toward each other under the stress from the ring. When the ring supported on the rollers whose shafts are thus tilted is rolled, the ring has its cross-sectional shape tapered.
When a plurality of rings having tapered cross-sectional shapes and slightly different circumferential lengths are fitted and laminated together into a laminated ring, it is difficult to combine the rings accurately because of the circumferential length difference present between the opposite circumferential edges of each of the rings. Therefore, the resultant laminated ring may not possibly be used as an endless belt for continuously variable transmissions. In addition, since the maraging steel used as the material of the rings is relatively expensive, the cost of produced laminated rings will become high if there are any rings that cannot be used as an endless belt for continuously variable transmissions due to the circumferential length difference.
One solution is to adjust the angles of the tilted shafts of the rollers which support the ring when the ring is to be rolled, for thereby minimizing the circumferential length difference that is present between the opposite circumferential edges of the ring. In order to accurately adjust the angles of the tilted shafts of the rollers, it is necessary to recognize precisely the circumferential length difference that is present between the opposite circumferential edges of the rolled ring. There has been a demand for a method of and an apparatus for accurately and easily measuring the circumferential length difference that is present between the opposite circumferential edges of a ring.
As described above, the circumferential length of a rolled ring is corrected for a target circumferential length after it has been subjected to a solution treatment to recrystallize the rolled structure. The circumferential length of the rolled ring is corrected by training the ring around a pair of rollers, rotating the rollers, and displacing a smaller-diameter roller disposed between the rollers in a direction perpendicular to the ring for thereby extending the ring.
The distance that the smaller-diameter roller is to be displaced is determined based on the circumferential length of the rolled ring and the desired circumferential length to be attained after the correction of the circumferential length of the rolled ring. The actual circumferential lengths of the rings after they have been subjected to the solution treatment are distributed in a considerably wide range due to variations of conditions in the rolling process and the solution treatment. Therefore, if the distance that the smaller-diameter roller is to be displaced is determined evenly based on the desired circumferential length to be attained by the rolled ring, then the corrected circumferential lengths of the rings are also distributed in a wide range. If the corrected circumferential lengths of the rings are distributed in a wide range, then many rings tend to fail to be fitted and laminated together into a laminated ring, and hence cannot be used as an endless belt for continuously variable transmissions. Consequently, the cost of produced laminated rings will become high if there are any rings that cannot be used as an endless belt for continuously variable transmissions.
It has been desired in the art to develop a method capable of correcting the circumferential length of a ring at a desired circumferential length.